In many radio communication standards, the signal to be transmitted is formatted in a succession of frames of fixed duration between which no signal is transmitted. Each frame generally begins with a sequence of symbols dedicated to the automatic gain control, hereinafter called AGC symbols, followed by a sequence made up of modulated information symbols.
The AGC symbols enable equipment receiving the transmitted signal to control the level or the power of the received signal so as to demodulate the symbols of the signal with a sufficient signal-to-noise ratio.
To produce the automatic gain control, the power of the transmitted signal over the duration of the AGC sequence has to be substantially constant. In other words, the aim is for the power variations at the start of frame not to be too great to be able to accurately measure the level of the signal. This objective is particularly important for the case of linear modulations for which the gain of the transmitting device as a function of the modulating signal is linear.
Now, when a frame is filtered by a shaping filter before its transmission, the influence of the filtering is such that the first information symbols, which are adjacent to the sequence of AGC symbols, also contribute to the portion dedicated to the automatic gain control in the signal actually transmitted (that is to say after shaping filtering).
Thus, the property sought, to obtain a signal power that is substantially constant at the start of each frame, is not observed.
One problem that the invention seeks to resolve is therefore to find a solution which makes it possible to minimize the amplitude variations of the signal at the start of each frame so as to enhance the accuracy of the power measurements for the automatic gain control.
Automatic gain control methods are notably described in the publications U.S. Pat. No. 6,985,099 and WO2003071695.
These solutions notably have the disadvantage of largely being unparameterizable and use, for the generation of the AGC symbols, heterogeneous modulations with respect to the information symbols. The modulations employed generally exhibit a peak power to average power ratio greater than one, which complicates the implementation of the automatic gain control.